This invention relates generally to non-invasive medical procedures and, in particular, to a method of analyzing retinal blood vessels to ascertain intracranial pressure non-invasively.
Monitoring of intracranial pressure is a of diagnostic and postoperative value for medical treatment of injured or diseased patients. This is especially true of individuals who have suffered closed head injuries, hemorrhagic strokes hydrocephalics and neurosurgery patients. Individuals suffering from such conditions are prone to experience brain swelling, infections, hemorrhages, edema and obstruction of cerebrospinal fluid ducts. Through the monitoring of intracranial pressure, potentially dangerous pressure changes can be relieved prior to attaining dangerous levels, and postoperative pressures can be monitored to assure a successful result or screening diagnostics can be performed which are indicative of such conditions.
Traditionally, intracranial pressure has been measured by insertion of a calibrated needle with pressure monitored as a function of the height a column of fluid rises within the needle as measured in millimeters mercury. While this method is both simple and direct, the invasiveness and discomfort associated with direct monitoring of intracranial pressure has led the art to explore non-invasive methods for monitoring intracranial pressure.
Non-invasive methods for monitoring intracranial pressure have taken advantage of various relationships between other physiological characteristics and intracranial pressure. For instance, measurement of a pressure volume index using ultrasonic blood flow transducers as described in U.S. Pat. No. 5,617,873; measurement of blood flow within the jugular vein following occlusion therein, as described in U.S. Pat. No. 4,204,547; measuring brain activity in response to sensory stimulus, as described in U.S. Pat. No. 4,564,022; pneumatic tonometry measurements, as described in Am. J. Dis. Child 137 (1983): 332; ultrasonic pulse probing, as described in U.S. Pat. No. 4,984,567; and listening to the skull cavity through the use of a microphone, as described in U.S. Pat. No. 4,008,711. While the relative merits of these various non-invasive approaches remains unsettled, a common feature of prior art methods for monitoring intracranial pressure is the adhesion of some type of sensor to a subject. Further, the prior art methods typically favor a subject to remain stationary in a supine position to obtain satisfactory measurements of intracranial pressure. Thus, a subject remains tethered to intracranial monitoring equipment necessitating a separate monitoring device for each subject. Thus, there exists a need for a method measuring intracranial pressure that does not require the adhesion of sensors to a subject, so as to facilitate periodic multiple subject monitoring with a single intracranial pressure monitoring device.
Normally, venous hemoglobin is 75 percent saturated with oxygen, thereby providing a reserve for critical states such as hemorrhage or heart attack, when more oxygen must be extracted by body organs. As a consequence, during bleeding states, capillary red blood cells upload more oxygen into tissues, resulting in a lower post-capillary venous oxygen saturation which may be detected and-used to assess the rate and quantity of internal hemorrhage over time.
Although the level of desaturated hemoglobin may be assessed invasively, as with catheters inserted into the subclavian vein or forearm vein, non-invasive procedures are preferred to minimize stress and infection. One such non-invasive technique is disclosed in U.S. Pat. No. 5,119,814, wherein a method and apparatus for monitoring blood loss via retinal venous oxygen saturation is provided to detect changes in the oxygen saturation of the retinal veins when internal bleeding in a patient occurs. The apparatus includes a retinal scanner that illuminates a plurality of points on the fundus and detects reflectance, and a signal processing means that uses reflectance spectroscopy techniques to convert the reflected signals into data points that can be stored or displayed. A decline in the value of these data points indicates a drop in venous hemoglobin saturation that allows the user to determine the rate and estimate the volume of blood loss.
Improvements to the system just described are disclosed in U.S. Pat. No. 5,308,919, wherein the optic disk region of the ocular fundus is illuminated with three or more wavelengths of light focused in approximately the same area. One of the light sources serves as a tracking beam which, through reflectance spectroscopy is used to determine when the tracking light beam is focused upon the optic disk. The scanning light beams are primarily focused within the boundary of the tracking beam. The intensity of light reflected from retinal venous and arterioles is detected, and the arteriole venous oxygen difference is determined.
Spectroscopic interrogation of the ocular fundus is known to the art to be a non-contacting method of obtaining vascular and ophthamologic information. For example, blood vessel dimensions and metering information is obtained through illumination of the fundus, as described in U.S. Pats. No. 5,090,799; 5,640,963 and 4,950,070. The prior art has heretofore not determined whether intracranial pressure is related to the flow characteristics of blood vessels within the ocular fundus.
The present invention provides a method for estimating intracranial pressure by the measurement of deoxyhemoglobin saturation obtained through retina scanning. A method of the present invention includes measuring a subject cardiac cycle while impinging with a light beam on a blood vessel within a subject retina. The light beam is used to measure physical characteristics of the blood vessel to determine subject deoxyhemoglobin saturation. Based on measurement of a subject intraocular pressure, cardiac cycle and the deoxyhemoglobin saturation, intracranial pressure is calculated. The method of the present invention is contrasted with prior art methods of non-invasively determining intracranial pressure in that scanning a subject retina while simultaneously measuring the subject cardiac cycle occurs independently of implanting or adhering a sensing device to a subject head or neck region. The determination of intracranial pressure based upon deoxyhemoglobin saturation within blood vessels of the retina is a novel aspect of the present invention. Other aspects and advantages will become apparent hereinafter.